High pressure conduits, such as oil and gas pipe lines have generally been constructed with conventional steel pipes. These pipelines are subject to both internal and external pressures. Internal pressure is required to transport the fluid or gases within the pipeline. External pressure is created by the weight of soil or water on the pipeline when the pipeline is embedded in the ground or submerged in water.
While steel pipes provide the requisite strength for withstanding the internal and external pressures, they have a high susceptibility to corrosion. A corrosive environment is fostered by contact between the internal foreign media (e.g., the liquids or gases being transported by the pipeline) and the steel, or by contact with external conductive foreign media and the steel. The external foreign media could be soil in cases where the pipe is buried under ground, or sea water in cases where the pipe is submerged in an ocean or water in cases where the pipe runs along sewer systems or exposed to rain. Corrosion decreases the pipe strength and may cause the pipe to leak or burst under pressure.
To overcome this disadvantage, steel reinforced composite pipes have been developed. These pipes have a wall of steel coated with a polymeric material, or of steel embedded in the fiber reinforced composite, such as a fiber glass resin system. The coating or resin system protects the steel from corrosion by shielding it from any contact with the foreign media. One example of steel reinforced composite pipe is disclosed in the Cocks Patent, No. 4,351,364, the subject matter of which is hereby incorporated by reference. The pipe disclosed therein is a structural wall section sandwiched between inner and outer linings. The linings are resin rich layers reinforced with glass or other fibers. The structural wall section is made of three or more structural steel reinforcing layers coated with structural epoxy resin. The individual layers of the pipe are successively built up, one upon the other, on a mandrel or pipe winding machining. Each lining layer is formed by helically winding resin wetted fiber rovings. Each steel layer is formed by helically winding a steel strip coated with resin. The steel layers are wound one on top of the other. Once "wound" the pipe is cured. This type pipe is commonly referred to as steel strip laminate pipe or "SSL pipe".
Typically the desired lap sheer strength between overlapping steel layers in SSL pipe is 1800 psi. Such a strength cannot be achieved by adhering one steel layer to the other using only a resin system. In order to achieve such strength, it is common to sandblast the steel prior to winding for forming the steel layers in the pipe.
Sandblasting creates a mechanical roughness on the steel providing a stronger mechanical/adhesive bonding. However, this bonding does not have long term durability under wet environments. Testing of a lap shear specimen consisting of sandblasted steel specimens bonded using EPON 826/IPD adhesive showed an 80% lap sheer strength loss after 505 hours of hot water exposure. Moreover, the aging of the sandblasted steel surface significantly reduces lap shear strength due to surface oxidation. Furthermore, sandblasting is a burdensome process. Typically, a 5000 horsepower air compressor would be required to sandblast production quantities of steel strips. A special facility would also be needed due to the dusty and noisy nature of sandblasting.
As such, a system is needed for bonding the steel strip formed layers to each other and to the SSL pipe linings that would avoid the need for sandblasting the steel strips.
It has also been discovered that steel strip layers, bonded to each other with only a resin have inadequate peel strength. The inadequate peel strength is believed to be caused by a variance in the bond thickness along the length of the steel strip forming the layer. Resin bond thickness control is typically not very accurate at production speeds.
Glass flake fillers are sometimes used in the resin to improve the shear strength between steel strips. Typically, glass flake fillers make up about 5 to 10% of the resin by weight. However, even with the use of glass flake fillers it is difficult to control the bond thickness between the strips at production speeds, thus, resulting in inadequate strip peel strength.
Another problem with the bonding of the steel strips to each other in SSL pipes is cracking of the resin matrix between the steel strips also resulting in reduction of the peel strength. A further problem is resin shrinkage which occurs during curing and also results in a decreased bond strength between the steel strips.
SSL pipes are typically subjected to hot and wet conditions and the bond between the steel layers often fails under a combined mode failure, i.e., a shear/peel-off failure. As such, it is important that the bonds between the steel layers maintain a significant percentage of their lap shear strength and peel off strength under hot and wet conditions. Moreover, in order to speed up the SSL pipe manufacturing process, it is desirable that the debulking time (i.e., the time required to remove excess resin between the steel strips forming the layers to be bonded) of any resin system used is relatively short so as to reduce manufacturing times and thereby manufacturing costs.
Thus, a resin system is needed which allows for controlling of the bond thickness between adjacent steel strip layers and between the steel strip layers and the linings of the SSL pipe at production speeds. Moreover, a resin system is needed with enhanced crack growth resistance when cured and which is subject to reduced shrinkage during curing. Furthermore, a resin system providing for improved peel and shear strengths between the steel strips of the SSL pipe in hot and wet conditions and which allows for reduced debulking times is desired.